Mechanoluminescent Devices, Articles, and Methods

ABSTRACT

Mechanoluminescent devices and articles, such as wearable articles, that include mechanoluminescent devices. The mechanoluminescent devices may have a lateral type architecture or a vertical type architecture. The mechanoluminescent devices may be sensors, including pressure sensors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/656,706, filed Oct. 18, 2019, which claims priority to U.S.Provisional Patent Application No. 62/753,437, filed Oct. 31, 2018, andU.S. Provisional Patent Application No. 62/747,964, filed Oct. 19, 2018,which are incorporated by reference herein.

BACKGROUND

Pressure sensing devices may have various potential applications, suchas artificial electronic skins, structural health monitoring, energyharvesting, ulcer detection, soft robotics, wearable technology, etc.The stiffness and/or thickness of conventional pressure sensors,however, can limit their use in these types of applications and others.

Currently available pressure sensors typically are based on capacitance,piezoelectricity, or resistivity. As a result, these sensors requireelectric power or the use of batteries during operation.Mechanoluminescent (ML) based sensors, however, do not require anyexternal power at a sensing location.

ML materials emit light in response to mechanical stress. AlthoughML-based devices have been used as stress sensors, structural healthmonitoring devices, dynamic load sensing devices, energy harvestingdevices in dark settings, most, if not all, of these devices requirephoto collector devices, such as a photomultiplier tube, spectrometer,or CCD camera. These components, however, typically are not integratedand usually require a component for light transfer, or a specializeddesign for capturing light from the ML source by a photo collector,which can limit their use in practical applications.

ML materials have been used for structural health monitoring byembedding the materials in composites. Both in situ TriboluminescentOptical Fiber (ITOF) sensors and Triboluminescent Optical Fiber SensingPatches utilize optical fibers as a transmission component. In thesetriboluminescent-based sensing systems, however, the transmission oflight from the sensing location is a disadvantage. Additionally oralternatively, the ML crystals are side coupled to an optical fiber. Asignificant amount of light typically cannot be transmitted through theoptical fiber, and, therefore, is lost. According to the principle ofrefraction, if a light ray strikes perpendicularly to a transparentobject (e.g., the optical fiber), then it will reflect. Thisdisadvantageous phenomenon may occur due to the side coupling in ITOFsensors.

Organic-inorganic perovskites may offer tunable optical absorption,large light absorption coefficients, high and balanced charge carriermobility, and/or long carrier diffusion length, which can make themattractive light absorbing materials for photodetectors.

There remains a need for efficient ways of utilizing materials thatexhibit mechanoluminescence to construct devices, such as sensitivepressure sensors. There also remains a need for ML devices that takeadvantages of one or more properties of perovskites, including theirlight absorbing capability. There also remains a need for sensors,including pressures sensors, that are flexible, stable, durable, and/orhave a low production cost.

BRIEF SUMMARY

Provided herein are devices, including sensors, that address one or moreof the foregoing needs. The mechanoluminescent devices may include aperovskite and an ML material. The devices may include compact,sensitive, and/or robust, yet flexible, pressure sensors. In someembodiments, the devices include an integrated 2D and/or 3Dorgano-halide perovskite, single crystal organo-halide perovskite,and/or a mixed cation perovskite. The mechanoluminescent-perovskiteflexible sensors described herein may not require a power source at thesensing location. In some embodiments, the sensors can be subjected torepeated loading and generate a distinct electrical current signalcorresponding to each loading cycle. One or more of these features maymake the devices herein suitable for use in articles, such as wearablearticles, prosthetics, etc.

In one aspect, mechanoluminescent devices are provided. In someembodiments, the mechanoluminescent devices have a vertical devicearchitecture. The devices having a vertical device architecture mayinclude an electrode; a first layer that includes a perovskite; acounterelectrode; and a second layer that includes a mechanoluminescentmaterial and a matrix material in which the mechanoluminescent materialis dispersed, wherein the first layer is arranged between the electrodeand the counterelectrode, and the counterelectrode is arranged betweenthe first layer and the second layer.

In some embodiments, the mechanoluminescent devices have a lateraldevice architecture. The devices having a lateral device architecturemay include an electrode; a first layer that includes a perovskite; anda second layer that includes a mechanoluminescent material and a matrixmaterial in which the mechanoluminescent material is dispersed; whereinthe electrode includes two or more discrete contacts, and wherein (i)the first layer is arranged between the electrode and the second layer,or (ii) the electrode is arranged between the first layer and the secondlayer.

In another aspect, articles that include a mechanoluminescent device areprovided. The articles may include wearable articles, such as clothing,or prosthetics, such as artificial skin.

In yet another aspect, methods of fabricating devices are provided, andthe methods may include efficient, low-cost manufacturing techniques.

Additional aspects will be set forth in part in the description whichfollows, and in part will be obvious from the description, or may belearned by practice of the aspects described herein. The advantagesdescribed herein may be realized and attained by means of the elementsand combinations particularly pointed out in the appended claims. It isto be understood that both the foregoing general description and thefollowing detailed description are exemplary and explanatory only andare not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts a schematic of an embodiment of a device having avertical architecture.

FIG. 1B depicts a schematic of an embodiment of a device having avertical architecture.

FIG. 1C depicts a schematic of an embodiment of a device having avertical architecture.

FIG. 1D depicts a schematic of an embodiment of a device having avertical architecture.

FIG. 1E depicts a schematic of an embodiment of a device having avertical architecture.

FIG. 1F depicts a schematic of an embodiment of a device having avertical architecture.

FIG. 1G depicts a schematic of an embodiment of a device having avertical architecture.

FIG. 2A depicts a side view of a schematic of an embodiment of a devicehaving a lateral architecture.

FIG. 2B depicts a top view of the device of FIG. 2A.

FIG. 2C depicts a side view of a schematic of an embodiment of a devicehaving a lateral architecture.

FIG. 2D depicts a side view of a schematic of an embodiment of a devicehaving a lateral architecture.

FIG. 2E depicts a side view of a schematic of an embodiment of a devicehaving a lateral architecture.

FIG. 2F depicts a cross-sectional view of the device of FIG. 2E.

FIG. 3 depicts the response (current) of an embodiment of a device topressure applied by hand 3 times with a bias voltage of 10 V (top), and4 times with a bias voltage of −5 V (bottom).

FIG. 4 is a schematic of a likely mechanism of an embodiment of aphoto-sensing material in an embodiment of a device.

FIG. 5 is a schematic of a cycle of an embodiment of a bending test.

FIG. 6A depicts the response of an embodiment of a sensor to twopressing rates.

FIG. 6B depicts the response of an embodiment of a sensor to verticaldisplacements of 0.6 mm and 1.2 mm.

FIG. 6C depicts the response of an embodiment of a sensor to pressureapplied by an index finger.

FIG. 7 depicts a band energy diagram of an embodiment of a device.

FIG. 8A depicts the response of an embodiment of a device to pressure.

FIG. 8B depicts a current versus time plot for the response of anembodiment of a sensor to applied pressures.

FIG. 8C depicts the response of an embodiment of a sensor to pressure.

FIG. 9 depicts the response of an embodiment of a sensor to a fingertouch.

FIG. 10A depicts the ML intensity of an embodiment of a composite thinfilm when stretched 5 times.

FIG. 10B depicts the ML intensity of an embodiment of a composite thinfilm when bent three times.

FIG. 11 depicts a schematic of an embodiment of a bendable device havinga vertical device architecture.

FIG. 12 depicts a schematic of an embodiment of a device having avertical device architecture.

FIG. 13 depicts a schematic of an embodiment of a device having avertical device architecture.

FIG. 14A depicts the response of an embodiment of a sensor tocontinuous, linearly applied pressure.

FIG. 14B depicts a current response versus time plot for an embodimentof a device.

DETAILED DESCRIPTION

Provided herein are mechanoluminescent devices that may be used as asensor, such as a pressure sensor. The devices provided herein mayinclude any combination of the elements herein, and the elements may bearranged according to any architecture. In some embodiments, the devicesherein have a vertical device architecture or a lateral devicearchitecture. When the devices are used in a sensor, a sensor, such as apressure sensor, may include devices of one or more architectures.

Vertical Device Architectures

The devices provided herein may have a vertical device architecture. Insome embodiments, the devices having a vertical device architectureinclude an electrode; a first layer that includes a perovskite; acounterelectrode; and a second layer that includes a mechanoluminescentmaterial and a matrix material in which the mechanoluminescent materialis dispersed. The first layer may be arranged between the electrode andthe counterelectrode, and the counterelectrode may be arranged betweenthe first layer and the second layer.

As used herein, the phrase “arranged between” does not connote that anytwo layers are necessarily in contact with each other; therefore, alayer that is “arranged between” two other layers may be in contact with(i) one of the other layers, (ii) both of the other layers, or (iii)neither of the other layers.

In some embodiments, the devices also include a substrate arrangedbetween the counterelectrode and the second layer. The substrate, asexplained herein, may be transparent, flexible, or a combinationthereof. As used herein, the term “transparent” refers to materialshaving a total transmittance of at least 90%, or at least 95%, or atleast 98%.

In some embodiments, the devices also include at least one of a firstcharge transporting layer and a first charge blocking layer, wherein thefirst charge transporting layer, the first charge blocking layer, orboth the first charge transporting layer and the first charge blockinglayer are arranged between the first layer and the counterelectrode. Thefirst charge transporting layer may be in contact with thecounterelectrode.

In some embodiments, the devices include at least one of a second chargetransporting layer and a second charge blocking layer, wherein thesecond charge transporting layer, the second charge blocking layer, orboth the second charge transporting layer and the second charge blockinglayer are arranged between the first layer and the electrode. The secondcharge transporting layer may be in contact with the electrode.

As used herein, the phrases “first charge” and “second charge”independently refer to (i) electrons and holes, respectively, or (ii)hole and electrons, respectively, for the charge transporting and chargeblocking layers. Therefore, [1] if a “first charge transporting layer”is an electron transporting layer, then the “second charge transportinglayer” is a hole transporting layer, and vice versa, and/or [2] if a“first charge blocking layer” is a hole blocking layer, then the “secondcharge blocking layer” is an electron blocking layer, and vice versa.Also, [1] if a “first charge transporting layer” is an electrontransporting layer, then a “first charge blocking layer” may be anelectron blocking layer or a hole blocking layer, because “first charge”and second charge” are selected independently for the chargetransporting and charge blocking layers.

In some embodiments, the devices also include a third layer thatincludes one or more reflector materials, wherein the second layer isarranged between the counterelectrode and the third layer.

FIGS. 1A-1G depict schematics of embodiments of devices having avertical architecture, but other arrangements of layers are envisioned.

FIG. 1A depicts a schematic of an embodiment of a device having avertical architecture. The device 100 of FIG. 1A includes an electrode110 and a counterelectrode 130. A first layer 120 that includes aperovskite is arranged between and in contact with both the electrode110 and the counterelectrode 130. A second layer 140 that includes amechanoluminescent material dispersed in a matrix material is arrangedadjacent to and in contact with the counterelectrode 130. In someembodiments, the counterelectrode 130 is transparent.

FIG. 1B depicts a schematic of an embodiment of a device having avertical architecture. The device 101 of FIG. 1B includes an electrode110 and a counterelectrode 130. A first layer 120 that includes aperovskite is arranged between and in contact with both the electrode110 and the counterelectrode 130. A substrate 150, which may be aflexible substrate, is arranged between and in contact with both thecounterelectrode 130 and a second layer 140 that includes amechanoluminescent material. In some embodiments, the counterelectrode130 and the substrate 150 are transparent.

FIG. 1C depicts a schematic of an embodiment of a device having avertical architecture. The device 102 of FIG. 1C includes an electrode110 and a counterelectrode 130. A first layer 120 that includes aperovskite is arranged adjacent to and in contact with the electrode110. A first charge transporting layer 160 is arranged between and incontact with both the first layer 120 and the counterelectrode 130. Asubstrate 150, which may be a flexible substrate, is arranged betweenand in contact with both the counterelectrode 130 and a second layer 140that includes a mechanoluminescent material. In some embodiments, thefirst charge transporting layer 160, the counterelectrode 130, and thesubstrate 150 are transparent.

FIG. 1D depicts a schematic of an embodiment of a device having avertical architecture. The device 103 of FIG. 1D includes an electrode110 and a counterelectrode 130. A first layer 120 that includes aperovskite is arranged adjacent to and in contact with the electrode110, and a first charge transporting layer 160 is arranged between andin contact with both the first layer 120 and the counterelectrode 130. Asecond layer 140 that includes a mechanoluminescent material is arrangedbetween and in contact with both the counterelectrode 130 and areflector layer 170.

FIG. 1E depicts a schematic of an embodiment of a device having avertical architecture. The device 104 of FIG. 1E includes an electrode110 and a counterelectrode 130. A first charge transporting layer 170 isarranged between and in contact with both the electrode 110 and a firstlayer 120 that includes a perovskite. A second charge transporting layer160 is arranged between and in contact with both the first layer 120 andthe counterelectrode 130. A substrate 150, which may be a flexiblesubstrate, is arranged between and in contact with both thecounterelectrode 130 and a second layer 140 that includes amechanoluminescent material. In some embodiments, the second chargetransporting layer 160, the counterelectrode 130, and the substrate 150are transparent.

FIG. 1F depicts a schematic of an embodiment of a device having avertical architecture. The device 105 of FIG. 1F includes an electrode110 and a counterelectrode 130. A first charge transporting layer 170 isarranged between and in contact with both the electrode 110 and a firstlayer 120 that includes a perovskite. A second charge transporting layer160 is arranged between and in contact with both the first layer 120 andthe counterelectrode 130. A second layer 140 that includes amechanoluminescent material is arranged adjacent to and in contact withboth the counterelectrode 130 and a reflector layer 180.

FIG. 1G depicts a schematic of an embodiment of a device having avertical architecture. The device 106 of FIG. 1G includes an electrode110 and a counterelectrode 130. A first charge transporting layer 170 isarranged between and in contact with both the electrode 110 and a firstcharge blocking layer 190. A first layer 120 that includes a perovskiteis arranged between and in contact with both the first charge blockinglayer 190 and a second charge blocking layer 180. A second chargetransporting layer 160 is arranged between and in contact with both thesecond charge blocking layer 180 and the counterelectrode 130. Asubstrate 150, which may be a flexible substrate, is arranged betweenand in contact with both the counterelectrode 130 and a second layer 140that includes a mechanoluminescent material. In some embodiments, thesecond charge blocking layer 180, second charge transporting layer 160,the counterelectrode 130, and the substrate 150 are transparent.

Lateral Device Architectures

In some embodiments, the devices provided herein have a lateral devicearchitecture. The lateral device architecture typically includes anelectrode that includes two or more discrete contacts disposed on alayer of a device.

In some embodiments, the devices having a lateral device architectureinclude an electrode; a first layer that includes a perovskite; and asecond layer that includes a mechanoluminescent material and a matrixmaterial in which the mechanoluminescent material is dispersed; whereinthe electrode includes two or more discrete contacts, and wherein (i)the first layer is arranged between the electrode and the second layer,or (ii) the electrode is arranged between the first layer and the secondlayer.

In some embodiments, the devices also include a substrate. In someembodiments, the second layer is arranged between the first layer andthe substrate when the first layer is arranged between the electrode andthe second layer. In some embodiments, the first layer is arrangedbetween the electrode and the substrate when the electrode is arrangedbetween the first layer and the second layer.

In some embodiments, the devices also include an encapsulation layer. Atleast a portion of the encapsulation layer may be in contact with theelectrode. The encapsulation layer generally may be formed of anymaterial, such as a transparent material, that is capable of preventingor reducing the likelihood of the electrode contacting a foreignsubstance, such as water.

In some embodiments, the devices also include a third layer thatincludes a reflector material, wherein the second layer is arrangedbetween the first layer and the third layer.

FIGS. 2A-2E depict schematics of embodiments of devices having a lateralarchitecture, but other arrangements of layers are envisioned.

FIG. 2A is a schematic depicting a side view of an embodiment of adevice having a lateral device architecture. The device 200 of FIG. 2Aincludes an electrode 210, a first layer 220 that includes a perovskite,and a second layer 230 that includes a mechanoluminescent material and amatrix material in which the mechanoluminescent material is dispersed.The first layer 220 is arranged between and in contact with both theelectrode 210 and the second layer 230. FIG. 2B is a top view of thedevice 200 of FIG. 2A. FIG. 2B depicts the first layer 220 and theelectrode 210.

FIG. 2C is a schematic depicting a side view of an embodiment of adevice having a lateral device architecture. The device 201 of FIG. 2Cincludes an electrode 210, a first layer 220 that includes a perovskite,and a second layer 230 that includes a mechanoluminescent material and amatrix material in which the mechanoluminescent material is dispersed.The first layer 220 is arranged between and in contact with both theelectrode 210 and the second layer 230. The device 201 also includes asubstrate 240. The second layer 230 is arranged on and in contact withthe substrate 240. A top view of the device of FIG. 2C is identical toFIG. 2B, which depicts a top view of the device of FIG. 2A.

FIG. 2D is a schematic depicting a side view of an embodiment of adevice having a lateral device architecture. The device 202 of FIG. 2Dincludes an electrode 210, a first layer 220 that includes a perovskite,and a second layer 230 that includes a mechanoluminescent material and amatrix material in which the mechanoluminescent material is dispersed.The first layer 220 is arranged between and in contact with both theelectrode 210 and the second layer 230. The device 201 also includes asubstrate 240, and a third layer 250 that includes one or more reflectormaterials. The third layer 250 is arranged between and in contact withboth the second layer 230 and the substrate 240. A top view of thedevice of FIG. 2D is identical to FIG. 2B, which depicts a top view ofthe device of FIG. 2A.

FIG. 2E is a schematic depicting a side view of an embodiment of adevice having a lateral architecture. The device 203 of FIG. 2E includesan electrode 210, a first layer 220 that includes a perovskite, a secondlayer 230 that includes a mechanoluminescent material and a matrixmaterial in which the mechanoluminescent material is dispersed, and anencapsulation layer 240. The electrode 210 is arranged between and incontact with the encapsulation layer 240 and the first layer 220. Theencapsulation layer 240 is adjacent to and in contact with the secondlayer 230. The first layer 220 is disposed on and in contact with asubstrate 250. A cross-sectional view of the device 203 of FIG. 2E isdepicted at FIG. 2F, which depicts the encapsulation layer 240 anddiscreet contacts of the electrode 210.

Mechanoluminescent (ML) Materials

Generally any material (e.g., crystal) exhibiting ML behavior may beused in the devices described herein. About 50% of known inorganiccrystals and about 30% of known organic crystals exhibit ML behavior.Therefore, the mechanoluminescent material may be selected from a widerange of inorganic and/or organic materials. A combination ofmechanoluminescent materials may be used to obtain longevity, improveddurability, higher ML yield, i.e., higher sensitivity, or a combinationthereof.

In some embodiments, the mechanoluminescent material includes particles.The particles may be of any size, but, in some embodiments, theparticles have an average largest dimension of about 2 μm to about 500about 2 μm to about 250 about 2 μm to about 100 or about 2 μm to about50 The “average largest dimension” may be determined by X-raydiffraction. In some embodiments, the particles are substantiallyspherical, and the “average largest dimension” is the “average largestdiameter”. In some embodiments, a single crystal, such as a singleEuD₄TEA crystal, having a largest dimension of about 2 mm to about 10cm, about 2 mm to about 5 cm, about 2 mm to about 1 cm, or about 2 mm toabout 5 mm can be used. The use of a single crystal as amechanoluminescent material may increase the sensitivity of a device.

The mechanoluminescent material may be selected based on its emissionwavelength(s), examples of which are provided at the following table.

ML Emission Wavelength of Different ML Materials.

ML materials Emission wavelength (nm) ZnS:Cu,Mn 457 and 587 ZnS:Cu,Pb520 SrAl₂O₄:Eu²⁺ 520 BaAl₂O₄:Eu²⁺ 505 Cu(NCS)(py)₂(PPh₃) 496 ZnS:Mn 585Pr³⁺-doped BaTiO₃-CaTiO₃ 618 LiNbO₃:Pr³⁺ 619

In some embodiments, the mechanoluminescent material includes (i) zincsulfide doped with copper, manganese, or a combination thereof, (ii)europium tetrakis dibenzoylmethide triethylammonium, or (iii) acombination thereof.

Zinc sulfide (ZnS) is an inorganic material that may exhibit highthermoluminescence (TL) emissions when doped with a small amount ofvarious metals, such as manganese and/or copper. The ML intensity ofcopper-doped zinc sulfide (ZnS:Cu) may increase as the pressure appliedto the material increases, without requiring any irradiation for MLrecovery. Due at least to this feature, the self-recovery of ML in theseand other crystals may take place by trapping drifting charge carriersin the presence of a piezoelectric field. Therefore, these crystals maybe suitable for use in a sensor, including a sensor designed fordurability.

Matrix Materials

Any matrix material in which a mechanoluminescent material may bedispersed can be used in the devices provided herein. In someembodiments, the matrix material is transparent, flexible, or acombination thereof.

In some embodiments, the matrix material includes polydimethylsiloxane(PDMS), poly(methyl methacrylate) (PMMA), polystyrene, polycarbonate,polyurethane (PU), polyvinylidene fluoride (PVDF), or a combinationthereof.

Any amount of a mechanoluminescent material may be dispersed in a matrixmaterial, and the mechanoluminescent material may be dispersed in thematrix material non-uniformly or substantially uniformly. In someembodiments, the second layer of the devices herein include a weightratio of a mechanoluminescent material to matrix material of about 0.1:1to about 1:0.1, about 0.3:1 to about 1:0.3, about 0.5:1 to about 1:0.5,about 0.8:1 to about 1:0.8, about 0.9:1 to about 1:0.9, or about 1:1.

Flexible Substrate

The substrates of the devices herein may include any material. In someembodiments, the substrate is transparent, flexible, or a combinationthereof.

In some embodiments, the substrate includes polyethylene terephthalate(PET), polydimethylsiloxane (PDMS), hydroxypropylcellulose (HPC), or acombination thereof.

Electrode/Counterelectrode

The electrode and counterelectrode of the devices herein may include anyconductive material. In some embodiments, an electrode and/orcounterelectrode includes indium tin oxide (ITO), In₂O₃/Au/Ag, Au, Ag, acarbon-based material, or a combination thereof. The carbon-basedmaterial may include carbon nanotubes, carbon nanofibers, or acombination thereof. In some embodiments, the carbon-based materialincludes a buckypaper.

When the devices herein have a vertical device architecture, theelectrode and/or the counterelectrode may be in the form of a layer.When the devices herein have a lateral device architecture, theelectrode may include two or more discreet contacts. The two or morediscreet contacts generally may have any configuration.

Perovskite Materials

Any perovskite that is capable of absorbing light may be used in thedevices described herein. In some embodiments, the perovskite includes aRuddlesden-Popper layered perovskite, an organo-metal halide perovskite,or a combination thereof. In some embodiments, the perovskite includes amixed cation perovskite. The perovskite may have any structure, forexample, the perovskite may have a 0D structure, a 1D structure, a 2Dstructure, a quasi-2D structure, a 3D structure, or a combinationthereof. The layers of the devices herein that include a perovskite mayinclude a matrix material in which the perovskite is dispersed, or thelayers may consist of the perovskite. The matrix material may beselected from those disclosed herein.

Non-limiting examples of perovskites and layers that may be used in thedevices herein are described at U.S. Pat. Nos. 9,896,462, 9,905,765,9,908,906, and U.S. Patent Application Publication No. 2017/0084848,which are incorporated herein by reference.

In some embodiments, the perovskite of the devices provided hereinincludes a Ruddlesden-Popper layered perovskite of the followingformula:

((BA)_(n-1)(MA)_(n-1)Pb_(n)I_(3n+1)) (where n is an integer).

A perovskite or class of perovskite may be selected based on theemission spectra of a mechanoluminescent material. For example, ifZnS:Cu is selected as a ML material, then a 2D Ruddlesden-Popper layeredperovskite n=3, 4 or 3D perovskite may be selected. Due to the fact thatthe bandgap energy of the ZnS:Cu is typically about 2.28 eV, the bandgapenergy of the perovskite materials, in some embodiments, is lower than2.28 eV. For example, the bandgap is (i) 2.03 eV for n=3Ruddlesden-Popper layered perovskites, (ii) 1.91 eV for n=4Ruddlesden-Popper layered perovskites, and (iii) 1.50 eV for 3Dperovskite (n=co). Similarly, a 3D perovskite (n=cc) may be selectedwhen EuD₄TEA is the ML material. EuD₄TEA has a ML emission intensitypeak at 617 nm.

A mixed cation perovskite may be used in the devices described herein.Not wishing to be bound by any particular theory, it is believed thatmixed cation perovskites, at least in some instances, may improve thestability of the devices described herein. Therefore, in someembodiments, the devices include a mixed cation perovskite, such as amixed cation perovskite of the following formula:

FA_(0.75)Cs_(0.25)Pb_(0.5)Sn_(0.5)X₃,FA_(x)MA_(1-x)PbX₃,

wherein X is a halide, and x is 0≤x≤1.

In addition, a single crystal perovskite, such as a single crystalperovskite of the following formula—

(C₄H₉NH₃)_(n)(CH₃NH₃)_(n-1)Pb_(n)I_(3n+1)(n=1, 2, 3, 4 and ∞).

In some embodiments, 2D, n=1; quasi-2D, n=2; and 3D, n=00 crystals aregrown and integrated with ML crystals to prepare embodiments of thedevices described herein. In some embodiments, a single crystalperovskite having no (or less) grain boundary (i.e., no potential dropalong the length and fewer defects) is about 1 mm to about 10 mm inlength, and may be used to achieve a high current yield per small amountof applied pressure.

Electron Transporting Layer

In some embodiments, the electron transporting material includes a metaloxide. Non-limiting of metal oxides include SnO₂, TiO₂, ZnO,[6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), or a combinationthereof. The following table depicts various embodiments of electrontransporting materials and their properties.

Electron Transporting Materials and their Properties.

Annealing E_(HOMO) E_(LUMO) E_(g) Material temperature (° C.) (eV) (eV)(eV) TiO₂ 450 −7.3 −4.1 3.2 SnO₂ 150-180 −8.1 −4.5 3.6 ZnO 220 −7.6 −4.23.4 PCBM  70 −5.92 −3.74 2.18

Hole Transporting Materials

Generally, any known hole transporting material may be used in the holetransporting layers of the devices provided herein. In some embodiments,the hole transporting material is an inorganic hole transportingmaterial, an organic hole transporting material, or a combinationthereof. In some embodiments, the hole transporting material is apolymeric hole transporting material. In some embodiments, the holetransporting material is a small organic molecule hole transportingmaterial.

In some embodiments, the hole transporting materials includeN²,N²,N^(2′),N^(2′),N⁷,N⁷,N^(7′),N^(7′)-octakis(4-methoxyphenyl)-9,9′-spirobi[9H-fluorene]-2,2′,7,7′-tetramine(Spiro-OMeTAD), polytriarylamine (PTAA), fluorine-dithiophene (FDT),Cu-phthalocyanine (CuPc), copper (I) thiocyanate (CuSCN), poly3-hexylthiophene (P3HT), poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS),poly[[(2,4-dimethylphenyl)imino]-1,4-phenylene(9,9-dioctyl-9H-fluorene-2,7-diyl)-1,4-phenylene](PF8-TAA), poly(9,9-dioctylfluorene) (PFO), polyaniline (PANI),poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)(PCPDTBT),N-(6-amino-2,4-dioxo-1-propylpyrimidin-5-yl)-N-(2-methoxyethyl)-2-phenylbutanamide(PDPP3T), or a combination thereof.

Reflector Layers

Generally, any reflective material may be used as, or in, the layersthat includes a reflector material described herein. In someembodiments, the layers include a thin layer of Au or Ag. These layermay maximize or increase the light harvesting by reflecting the lightnot traveling toward a perovskite of the devices.

Articles and Applications

In some embodiments, the devices herein are sensors having a currentintensity that increases linearly as the pressure applied to the devicesincreases. In some embodiments, the sensors described herein can detectpressure at 25 kPa with a sensitivity of 0.0115 μA/kPa, have a fastresponse of time of <25 μs, a relatively large sensing range (e.g.,about 25 kPa to about 475 kPa), or a combination thereof. The sensorsmay display consistent signals over a number of cycles, (e.g., over 100cycles, 1000 cycles, or more). The sensors, for example, have thepotential to (i) detect one or more pathologies (e.g., ulcers on aperson's foot), (ii) perform structural health monitoring, (iii) providereliable and/or sensitive touchpad user interfaces, or (iv) acombination thereof.

Articles are provided that include at least one of the devices providedherein. The devices, in some embodiments, are sensors, such as pressuresensors. The devices, therefore, may be a part of any article that wouldbenefit from the presence of a sensor, such as a pressure sensor.

In some embodiments, the articles are wearable articles. The wearablearticles may include any one or more of the devices described herein.Examples of wearable articles include an article of clothing (e.g.,shirt, socks, pants, coats, etc.), an accessory (e.g., a watch, wristband, necklace, bracelet, head band, etc.), a shoe, or a combinationthereof.

In some embodiments, the articles are prosthetics. The prosthetics mayinclude one or more of the devices described herein. The prosthetics mayinclude artificial skin, an artificial limb, or a combination thereof.

In the descriptions provided herein, the terms “includes,” “is,”“containing,” “having,” and “comprises” are used in an open-endedfashion, and thus should be interpreted to mean “including, but notlimited to.” When methods and devices are claimed or described in termsof “comprising” various components or steps, the devices and methods canalso “consist essentially of” or “consist of” the various components orsteps, unless stated otherwise.

The terms “a,” “an,” and “the” are intended to include pluralalternatives, e.g., at least one. For instance, the disclosure of “amatrix material,” “a perovskite,” “an electrode,” and the like, is meantto encompass one, or mixtures or combinations of more than one matrixmaterial, perovskite, electrode, and the like, unless otherwisespecified.

EXAMPLES

The present invention is further illustrated by the following examples,which are not to be construed in any way as imposing limitations uponthe scope thereof. On the contrary, it is to be clearly understood thatresort may be had to various other aspects, embodiments, modifications,and equivalents thereof which, after reading the description herein, maysuggest themselves to one of ordinary skill in the art without departingfrom the spirit of the present invention or the scope of the appendedclaims. Thus, other aspects of this invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein.

Example 1—Device Having Lateral Architecture

A device having a lateral architecture was fabricated in this example.The device of this example was a fully integratedmechanoluminescent-perovskite flexible and bendable pressure sensorhaving a lateral structure according to the schematic of FIG. 2C. Thelayers of the device of this example included polyimide (210 of FIG.2C)/ZnS:Cu-PDMS (230 of FIG. 2C)/perovskite (220 of FIG. 2C)/Au (210 ofFIG. 2C).

A polyimide substrate was treated with plasma cleaning, and onto thesubstrate the ZnS:Cu/PDMS composite was deposited using spin coating.

PDMS was cured on a hot plate at 150° C. for 5 minutes. The top surfaceof the ZnS:Cu/PDMS was treated with plasma cleaning to make the surfacehydrophilic. A 2D Ruddlesden-Proper perovskite (n=3) was thenspin-coated on the ZnS:Cu/PDMS layer with 2000 rpm for 45 seconds, andbaked on a hot plate for 5 minutes. A 80 nm gold coating was depositedon the perovskite layer.

Spin-coating the perovskite on the PDMS/ZnS:Cu film surface wasdifficult, likely because of the roughness of the top surface. The highroughness of the top surface of the PDMS/ZnS:Cu probably stemmed fromthe relatively large ZnS:Cu particles (2-30 μm particle size).

Therefore, another thin layer of bare PDMS was spin-coated on thePDMS/ZnS:Cu film to provide a smoother surface. As the device of thisexample had a thin flexible, bendable polyimide substrate, the devicewas flexible and bendable. This device could be used for wearabletechnology, such as e-skin for health monitoring.

The sensors of this example were tested by applying pressure with theindex finger. The output current from the sensor was collected with aKEITHLEY™ 2410 source meter (TEKTRONIX™, USA) and a custom-builtLABVIEW™ program (NATIONAL INSTRUMENTS™, USA).

FIG. 3 displays the sensor response (current) to applied pressure byhand (soft touch by index finger); pressure (top) was applied on thesensor 3 times with a bias voltage of 10 V, and (bottom) 4 times with abias voltage of −5 V. These results demonstrated that the lateral designof the sensor of this example had the potential to serve as a highlysensitive, flexible, and/or bendable pressure sensor. The flexibilityand/or bendability of the device was attributed, at least in part, tothe substrate (a thin polyimide substrate having a thickness of about100 μm).

Example 2—Device Having Lateral Architecture

In this example, a pressure sensor was fabricated that included twofunctional layers: i) a ZnS:Cu/PDMS composite as a light source, and ii)a perovskite as a photoactive layer.

The pressure sensors of this example had a structure according to FIG.2E and FIG. 2F, and were fabricated, as explained in detail below, bysequentially depositing perovskite layer (220 of FIG. 2E), PMMA (240 ofFIG. 2E, FIG. 2F), and ZnS:Cu-PDMS (230 of FIG. 2E) composite layer onflexible PET substrates (250 of FIG. 250 ). The ultra-thin, highlytransparent PMMA layer allowed the transmission of ML light without, orwith very little, loss.

ML light was emitted by ZnS:Cu particles in response to an appliedpressure or strain, and was transmitted through a translucent PMMA laterto the perovskite layer.

FIG. 4 is a schematic of the likely mechanism of the photo-sensingmaterial as a hole-only device, wherein the electrons are trapped andholes are mobile.

PET substrates were successively cleaned with detergent, nano purewater, acetone, and isopropanol. In order to improve the wettability,the PET substrates were treated with oxygen plasma for 5 minutes.

The light-absorbing MAPb(Br_(0.1)I_(0.9))₃ perovskite films of thisexample were prepared using a one-step deposition technique referred toas a solvent-solvent extraction method. The precursor solution wasprepared by mixing CH₃NH₃I into 0.5 mL N-Methyl-2-pyrrolidone (NMP) and0.1 mL γ-butyrolactone (GBL).

A mixture of PbBr₂ and PbI₂ was poured into the previously preparedsolution and mixed on a hot plate at 65° C. for 2 hours. The solutionwas heated on a hot plate at 70° C. for 30 minutes. The perovskiteprecursor solution was spin-coated.

The perovskite-coated substrates were quickly immersed into diethylether (C₂H₅OC₂H₅, DEE) bath for 2 minutes. Within this time frame, NMPsolvent extraction and a complete crystallization occurred, resulting ina uniform, ultra-smooth perovskite film. The NMP is highly miscible indiethyl ether, therefore, DEE selectively extracted NMP solvent from thedeposited perovskite thin films, and left devoid areas where perovskitecrystallization rapidly occurred.

The perovskite was insoluble in diethyl ether, therefore, the DEE didnot dissolve crystallized films. The perovskite films were thermallyannealed at 70° C. for 5 minutes, and then 130° C. for 15 minutes. Atopthe perovskite layer were 100 nm thick Au electrodes. 30 mg of PMMA wasdissolved in 1 mL of chlorobenzene and the solution was spin-coated at3000 rpm for 30 seconds on the perovskite layer. The micro-sized ZnS:Cuwas thoroughly mixed in PDMS elastomer with a weight ratio of 2:1 withthe help of a mixer that used planetary motion to achieve thoroughmixing).

The ZnS:Cu/PDMS composite was spin-coated on the PMMA layer. The castedthin film was heated to cure it completely. The PMMA layer protected theperovskite layer from large ZnS:Cu particles (e.g., scratching by theparticles) during spin-coating. In addition, the PMMA prevented moistureinclusion to the perovskite. PDMS acted as another protective layer forthe device to protect it from (i) mechanical damage that can occurduring to scratching, dropping, etc., and/or (ii) further moistureinclusion into the perovskite layer.

Perovskite and ZnS: Cu Characterizations: UV-vis absorption spectra wererecorded on a CARY™ 5000 UV-vis FM-NIR spectrophotometer (Agilent, USA)in the 400 nm-850 nm wavelength range at room temperature. A confocalRaman system (Renishaw Confocal Microscope, USA) was used to collect thephotoluminescence emission spectra of ZnS:Cu crystals.

The X-ray powder diffraction (XRD) data on the ZnS:Cu powder wascollected using a PANALYTICAL X-PERT™ Pro Powder XRD machine.Transmission electron microscopy (TEM) was used to analyze the structureof ZnS:Cu crystal. Scanning electron microscopy (JEOL 7401high-resolution field emission SEM) was used to characterize themorphologies of perovskite thin films and ZnS:Cu particles. XRD patternsof thin perovskite films were collected using a SCINTAG™ XRD PowderDiffractometer with Cu Kα radiation (λ=1.5406 Å).

The time-resolved photoluminescence was performed at room temperatureusing an EDINBURGH™ FS5 spectrometer. Samples were excited by picosecondpulsed light emitting diode (EPLED-365) with an excitation wavelength of470 nm. The time-resolved photoluminescence was measured using timecorrelated single-photon counter, excited by a picosecond pulsed diodelaser (EPL-470 nm). ML emissions from ZnS:Cu crystals were recorded withan H-micro series spectrometer (C12880MA). An atomic force microscopy(AFM) instrument from Veeco Instruments Inc., USA was used tocharacterize the surface morphologies of perovskite.

Device characterization and testing: The current-voltage (I—V)characteristics of the photodetectors were measured by using aKEITHLEY™-2410 source measure unit (SMU). The mechanical bending testingwas conducted using a SHIMADZU™ mechanical testing machine.

The perovskite was a core material for the sensor of this example;therefore the optical properties of the perovskite played an importantrole in determining the sensor performance. A UV-vis spectrum of theperovskite of this example showed a range of absorption from 400 to 770nm, which demonstrated that the perovskite material of this exampleharvested ML light from the ML crystals of this example, and could do sofor almost all organic and inorganic ML crystals.

Many of the ML materials disclosed herein have ML emission wavelengthsof about 450 nm to about 650 nm. A band gap determination from a Taucplot was performed. The optical band gap (E_(g)) of theMAPb(Br_(0.1)I_(0.9))₃ perovskite was calculated to be 1.64 eV byextrapolating the linear portion of the (αhv)² versus photon-energyplot. The optical bandgap is the threshold for emitted photons to beabsorbed.

An ML spectrum of ZnS:Cu was collected, and a stable and strong greenemission band centered at 543 nm was detected. ML emission of ZnS:Culikely emerges from the recombination of the impurity-induced shallowdonor state and the t₂ state of Cu. The energy for an emitted photon canbe calculated from the following Equation (1):

$\begin{matrix}{E = {\frac{hv}{\lambda} \approx \frac{123{9.8}}{\lambda}}} & (1)\end{matrix}$

wherein E is the energy of the photon (eV), h is the Planck constant(4.136×10⁻¹⁵ eV·s), v is the speed of light (3×10⁸ ms⁻¹), and λ is thewavelength of the light. The calculated photon energy for ZnS:Cu was2.28 eV, corresponding to the peak emission wavelength of 543 nm, whichwas much higher than the optical band gap of the perovskite (1.64 eV).

In order to investigate the crystalline structure of the ZnS:Cuparticles, X-ray diffraction (XRD) and TEM were performed. The powderX-ray diffraction data collected from the as-received crystalsdemonstrated the presence of a cubic structure of ZnS:Cu, wherein thecubic phase was depicted with peaks at 28.57° (111), 47.49° (220), and56.37° (311). No other diffraction peak of impurity was observed in theXRD patterns. Regarding the peak at 28.57° (C(111)), the full width athalf maximum (FWHM) was 0.12296°. High-resolution transmission electronmicroscopy (HRTEM) images of ZnS:Cu were collected, along with an areaelectron diffraction (SAED) pattern, which quantitatively confirmed thecrystalline nature of the crystal. Higher crystallinity of crystalslikely resulted in enhanced ML intensity.

Scanning electron microscopy (SEM) was performed on the as-received MLpowder material and a SEM image of ZnS:Cu particles was collected. Theaverage particle size of the ZnS:Cu particles was about 8.91 μm and therange of the particle size was about 2.5 μm to about 29 μm.

Energy dispersive X-ray (EDX) spectra showed the elements present inZnS:Cu. The EDX analysis showed that Zn, S, and Cu elements were presentin ZnS:Cu, with no other impurities. SEM micrographs of the ZnS:Cu/PDMScomposite thin film exhibited ZnS:Cu particles surrounded by the PDMSmatrix. A pictures of the device under 365 nm UV light demonstrated thatthe ZnS:Cu emitted light of a bright green color. A 488 nm laser-excitedPL emission of ZnS:Cu was collected, and it included a strong greenemission band centered at 543 nm.

High-quality perovskite films contributed to the performance of thedevice of this example. The processing of perovskite had a significanteffect on film morphology, uniformity, and crystallinity of theperovskite films, resulting in a compelling impact on the deviceperformance. In order to characterize the perovskite film'scrystallinity and surface morphology, XRD and SEM data were collected.SEM images revealed compact perovskite thin films that completelycovered a surface of the PET substrate. SEM images confirmed thepinhole-free surface morphology and large grain size of theMAPb(Br_(0.1)I_(0.9))₃ perovskite of this example.

The average thickness of the perovskite film was about 500 nm, which, inthis example, was sufficient for carrier generation, extraction, andcapturing visible light. The average grain size of the perovskite was420 nm with a range of about 50 nm to about 1200 nm. Larger grain sizesin perovskite films typically possess fewer grain boundaries, which canresult in reduced defect density. When charge carriers encounter lesstrapping in grain boundaries, the efficiency of the device may improve.

In addition, the film surface was characterized using AFM, and theaverage roughness was about 12.6 nm in a typical scanning area of 5.0μm×5.0 μm. The three-dimensional AFM images further demonstrated thehighly smooth surface of the perovskite film. The fabrication of ahigh-quality, highly crystalline perovskite thin film on PET substratewas confirmed by the X-ray diffraction (XRD) pattern. The breakregion)(16-28.4° in the XRD pattern seeks to remove the strongdiffraction from the PET substrate. XRD peaks from perovskite crystalincluded three dominant crystallographic planes (100), (200), and (210),which were observed at the peak of diffraction angles of 14.2°, 28.6°,and 32°, respectively. These results confirmed the cubic halideperovskite crystalline structure.

In addition to peaks originating from the perovskite crystal, a peakobserved at 2θ=12.75° was the diffraction peak corresponding to the PETsubstrate, because the XRD pattern of a bare PET substrate exhibited thesame peak. The lead iodide (PbI₂) had a peak at 2θ=12.65°, correspondingto (001) plane, which was lower than, but relatively close to theobserved peak at 12.75°.

In order to further confirm whether there was any unconverted PbI₂ leftin the perovskite film of this example, the same perovskite wasfabricated on fluorine-doped tin oxide (FTO) glass. The XRD pattern ofthe perovskite on FTO glass did not include a peak at 20=12.75°.Therefore, it appeared that no unconverted PbI₂ was left in theperovskite. This was noteworthy, because the presence of PbI₂ in aperovskite can deteriorate device stability. To qualitatively analyzethe degree of crystallinity of the perovskite film, the full width athalf maximum (FWHM) of the C(100) peak of the perovskite film wascalculated to be 0.1974°. The crystallite size of the perovskite filmwas estimated from the C(100) peak using Scherrer's equation as shownbelow.

$\begin{matrix}{D = \frac{K\lambda}{B\cos\theta}} & (2)\end{matrix}$

wherein D, K, λ, B, θ is the crystallite size (nm), Scherrer constant,X-ray wavelength (nm), FWHM (radian), and XRD peak position (degree),respectively. The calculated crystallite size was 41.51 nm.

These results further indicated that the perovskite film had bettercrystallinity and fewer defects, which can contribute to altering, andpossibly improving, the photophysical properties of the device. Thesolvent-solvent extraction method contributed, in this example, to thefabrication of high-quality, ultra-smooth, highly crystalline perovskitefilms.

Characterization of Photodetector: In order to investigate the lightresponse ability of the device of this example, devices were fabricatedwithout a ZnS:Cu-PDMS layer on flexible PET substrates. These devicesacted as a flexible photodetector with an structure ofPET/perovskite/Au/PMMA. The current-voltage (I-V) curves were measuredin the dark and under light illumination. The I-V curves demonstrates alinear and symmetrical I-V relationship, which indicated an ohmicphotoconductive behavior.

The result indicated an ohmic contact between the as-preparedMAPb(Br_(0.1)I_(0.9))₃ perovskite and the Au electrodes, whichfacilitated the extraction of the photogenerated charge carriers. When aperovskite absorbs photons, charge carriers were created, which cause anincrease in conductivity. The valence band of the halide perovskite(˜5.3 eV) was near enough to the work function of the Au electrodes (5.1eV). Therefore, it had a 1.35 eV barrier for the electrons and a 0.2 eVbarrier for the holes. As such, the band configuration indicated thatthe device of this example operated as a single carrier device. Thedevice of this example acted as a hole-dominant device, becauseovercoming a barrier of 1.35 eV is unlikely, or not possible, forelectrons. The electrons were trapped, as depicted at FIG. 4 , and theholes easily flowed, upon injection, from one Au electrode to the other.

A small, or smallest, dark current may be advantageous for effectivelycapturing low ML light. I-V curves were collected at different lightintensities, where the dark current was small. Under an applied voltageof 10 V, the current outputs of the device of this example under darkand light conditions were 2.7×10⁻¹% and 6.35×10⁻⁷ A, respectively. Assuch, the light on/off ratio was >10³, which demonstrated that thedevice had a good light-switching behavior.

I-t curves were collected for different bias voltages. The curveinstantly elevated upon light illumination, and sharply declines whileswitched off. The low dark current and high on/off ratio indicated thatthe device of this example can be used for harvesting low ML light.

In order to measure the lifetime, time-resolved photoluminescence (PL)spectroscopy was performed on the perovskite. A time-resolved PLspectrum of perovskite material revealed that the material exhibitedfast (τ_(f)) and slow decay (τ_(s)) lifetimes of 2.27 ns and 39.4 ns,respectively. The lifetime constants were obtained by a stretchedbi-exponential curve fitting function. The fast and slow decay lifetimeconstants were related to the surface and bulk defects of the perovskitematerial. The trap density was calculated using space charge-limitedcurrent method as follows:

n _(traps)=2ε_(r)ε₀ V _(TFL) /eL ²

wherein e is the elementary charge, L is the thickness of the perovskitefilm, ε_(r) is the relative dielectric constant of the perovskitematerial, and ε₀ is the vacuum permittivity. Based on the I-V curve, thetrap filled limit voltage, V_(TFL) was 0.7 V, and the correspondingcalculated trap density was 9.28×10¹⁵ cm⁻³. The low trap density in theperovskite films likely contributed to high extraction of chargecarriers in the device of this example.

Sensor Response: In order to investigate the sensor response tomechanical bending, the sensor of this example was loaded vertically, asdepicted at FIG. 5 . A repeated bending test was performed that included100 cycles of a vertical elastic deformation of 1.2 mm. The sensorgenerated distinct signals for each cycle. The current data showed peaksignal consistency over the cycles. An analysis of the first 10 cyclesdemonstrated that the sensor output was consistently followed by themechanical input.

In order to investigate the sensor response with the pressing rate,sensors were displaced vertically. Data was collected regarding thesensor's response to various pressing rates. For a particulardisplacement of 0.6 mm, sensors were tested at frequencies of 0.55 Hzand 1.1 Hz, as depicted at FIG. 6A. The sensors' output signal increasedwith the increase of pressing rate. Data regarding the sensors' responseto different displacements (0.6 mm, 1.2 mm) at a frequency of 0.55 Hz.The current output increased with higher displacement, as depicted atFIG. 6B.

The sensors were attached to a carbon fiber composite for a 3-pointbending test. The test was performed at 1 Hz for 450 cycles. The sensorresponse (current) over time (0-400 s) for 450 cycles was plotted. Theapplied bias voltage was −10 V. The sensor generated distinct signalsfor each cycle. The sensor displayed consistent signals over cycles,which implied that the sensor can be used for structural healthmonitoring of large composite structures.

Also plotted was the sensor response to pressure applied by an indexfinger on the ML-perovskite sensor. The pressure was applied gently onthe sensor 15 times and the ML-perovskite sensor demonstrated signalseach time, as depicted at FIG. 6C. When the applied pressure was 20 kPa,the current output from the sensor was about 125 nA at a bias voltage of10 V. All tests were performed in ambient conditions (21° C., 70±1% RH).As such, the sensor has the potential to perform under high humidity.

The devices of this example represent an efficient way of harvesting MLlight with a low-temperature solution-processed mixed halide perovskite.The pressure sensors of this example have the ability to achievereal-time pressure sensing, and may have a number of potentialapplications, such as foot ulcer detection, structural healthmonitoring, artificial/prosthetic skins, and/or touchpad.

Example 3—Vertical Type Device

In this example, a flexible pressure sensor was fabricated, whichincluded a 2D Ruddlesden-Popper perovskite. This perovskite had arelatively robust material stability. The device architecture was avertical type structure, which likely contributed, at least in part, toits comparatively easy fabrication and efficient ML harvesting.

Unlike other conventional pressure sensors, the ML-perovskite flexiblepressure sensor of this example did not require any power at the sensinglocation.

The device of this example had a vertical architecture, similar to thearchitecture depicted at FIG. 1C, and included the following layers:(ZnS:Cu-PDMS/PET/In₂O₃—Au—Ag/bl-TiO₂/perovskite/Au).

The device included a thin flexible ZnS:Cu/PDMS film on a transparentpoly(ethylene terephthalate) (PET) substrate and a 2D Ruddlesden-Popperlayered perovskite ((BA)₂(MA)₂Pb₃I₁₀). A band energy diagram of thedevice is depicted at FIG. 7 .

ZnS:Cu crystals emitted ML light at 543 nm, i.e., 2.28 eV.(BA)₂(MA)₂Pb₃I₁₀ had a band gap energy of 2.05 eV, which was lower than2.28 eV. As such, in this example, (BA)₂(MA)₂Pb₃I₁₀ was used as anabsorbing layer for the device. The integrated structure was effectivefor sufficiently absorbing ML light from ZnS:Cu with minimumtransmission loss.

A highly uniform perovskite active absorber layer lay on a hole blockinglayer tin (IV) oxide (SnO₂) layer. Atop the perovskite layer was a 100nm thick Au as an active photocathode. The emitted light from the ZnS:Cucrystals passed through the transparent PET substrate to the perovskitelayer.

Micro-sized copper-doped zinc sulfide (ZnS:Cu) was thoroughly mixed witha polydimethylsiloxane (PDMS) elastomer with the aid of a mixer. TheZnS:Cu/PDMS composite was spin-coated on the non-conductive side of aflexible polyethylene terephthalate (PET) substrate to obtain a thinfilm. The casted thin film was baked on a hot plate to complete curing.

In order to prevent direct contact between the conductive PET substrateand the hole-conducting layer, a (few nano-meters thick) hole blockinglayer (bl-SnO₂) was casted onto the In₂O₃/Au/Ag-coated conductive sideof the flexible PET substrate with a high-speed spin-coating at 4500rpm, and heated at 150° C. for 15 minutes. On top of bl-TiO₂, theperovskite thin film was grown with a spin-coating method, and heated to100° C. for 5 minutes in a nitrogen-filled glove box. The (BA)₂MAPbI₃precursor (n=3) was synthesized by preparing MAPbI₃ precursor solutionsof different densities in a glovebox by dissolving (BA)₂(MA)₂Pb₃I₁₀ in amixed solvent of DMF and DMSO. A 100 nm layer of Au was deposited bythermal evaporation using a shadow mask to pattern the electrode.

Device characterization and testing: The ML emission from the ZnS:Cucrystals was recorded with an H-micro series spectrometer (C12880MA).Mechanical testing of the ML-perovskite sensor was performed by using aDMA Q800 instrument (TA Instruments, USA). The cyclic 3-point bendingtest was performed by utilizing a SHIMADZU™ mechanical testing system.The output current from the sensor was collected with a KEITHLEY™ 2410and a custom-built LAB VIEW™ program.

To measure the response time of the sensor, an amplifier (C7319) wasused to convert current-to-voltage, and a NI-6210 DAQ istrument wasconnected to the amplifier to collect data. Ultraviolet-visibleabsorption spectra were recorded on a CARY™ 5000 UV-VisFM-NIRspectrophotometer (Agilent, USA) at a wavenlength range of about 350 nmto about 800 nm at room temperature.

Scanning electron microscopy (JEOL 7401 field emission SEM, USA) wasused to characterize the surface and cross-section of films. An atomicforce microscope was used to perform Kelvin probe force microscopy(KPFM) and surface topography. Surface potential (SP) images wereacquired with a sensor tip in lift mode where topography was recorded inthe first pass and surface potential in the second pass. The cantileverresonance frequency was 57.5 kHz.

Device materials characterization: SEM images of a ZnS:Cu/PDMS film werecollected. ZnS:Cu particles were well dispersed in the PDMS polymer.Also measured was the transmittance of a PET/Au/Ag substrate, and at 543nm wavelength, the PET/Au/Ag substrate had a transmittance of 83%. As aresult, the device of this example had only a 17% transmission loss.

SEM was used to characterize the compactness of the perovskite layer.SEM images of perovskite films of various precursor concentrationsrevealed that the grain size increased with the increase of precursorconcentration. Under the scanning electron microscope, a 300 mgml⁻¹concentration film seemed less defective, compact, and containedrelatively large crystals.

In order to investigate the crystallinity of the perovskite films, X-raydiffraction was performed using an X-ray diffractometer (Cu Kα). Thedata were collected with a step size of 0.02°. The breakregion)(17-28.3° in the XRD pattern sought to remove the strongdiffraction from the PET substrate. The intensity increased with theincrease in precursor concentration. Three dominant crystallographicplanes ((111), (202), and (313)) were observed at the peak ofdiffraction angles of 14.3°, 28.68°, and 43.46°, which confirmed thepresence of (BA)₂(MA)₂Pb₃I₁₀ (n=3) perovskite phase.

ML emission spectra of ZnS:Cu also was collected. The micro-sized ZnS:Cucrystals emitted ML light at the peak of 543 nm with a range of 475-630nm. UV-Vis absorption spectra of various precursor concentration filmsalso were collected. The highly concentrated perovskite precursorcreated a thick film, which had higher UV-Vis absorption. The 400 mgml⁻¹sample had higher UV-Vis absorption compared to others. However, thickfilms generally had a higher probability of containing defects.

AFM topography and surface potential of perovskite films prepared fromdifferent precursor concentrations were collected. Topography imagesrevealed that the prepared perovskite films had grain sizes varying fromabout 2.35 μm to about 3.07 μm as the perovskite precursor concentrationincreased from 100 mgml⁻¹ to 400 mgml⁻¹.

The average grain size obtained for 100, 200, 300, 400 mgml⁻¹ were2.35±0.21, 2.75±0.52, 2.80±0.29 and 3.07±0.63 respectively. Theperovskite film prepared from 100 mgml⁻¹ and 200 mgml⁻¹ precursorconcentration showed some voids between perovskite crystals. However, byincreasing the concentration to 300 mgml⁻¹, uniformity and bettercoverage of the surface was obtained.

Therefore, a lesser number of total grain boundary area was obtained forperovskite film prepared with 300 mgml⁻¹ precursor concentration. Theincrease in grain boundary reduced the photocurrent, and likely acted asa recombination center for generated electron-hole pairs. The reductionin total grain boundary area helped to increase the charge transport asthe grain boundary acted as a carrier trapper due to incomplete atomicbonding and a large number of defects.

Kelvin probe force microscopy (KPFM) images of the perovskite filmsprepared from different precursor ratios exhibited higher surfacepotentials at grain boundaries (GB s) than within grains. Therefore, theminority carriers, i.e. electrons in p-type absorber layer wereattracted towards GB s, which agreed with other research. The surfacepotential mapping of the perovskite film with 300 mgml⁻¹ concentrationshowed less variation in potential (˜0.025 V-0.030 V) compared to 100mgml⁻¹(˜0.030 V to 0.028 V), 200 mgml⁻¹ (˜0.064 V-0.052V) and 400mgml⁻¹(˜0.119 V-0.098 V). This showed the presence of uniform phaseswhere the perovskite acted as a dominating phase in the film preparedfrom 300 mgml⁻¹ precursor without any secondary phases.

Also plotted was the relationship between perovskite precursorconcentration and maximum GB potential, dopant density, and density ofcharged trap states. The dopant density and density of trap states werecalculated using a grain boundary model from surface potential imagesobtained using KPFM. In this model, a grain boundary corresponded to asurface with a surface charge. Therefore, the net doping density(P_(net)) of perovskite could be calculated from the barrier height ofband bending Δφ_(gb) and the space charge region (SCR) width (w). Thedoping density (P_(net)) and trap charge states (P_(gb,trap)), are givenby the following equations:

$P_{net} = \frac{2\varepsilon_{0}\varepsilon_{r}{\Delta\varphi}_{gb}}{e^{2}w^{2}}$$P_{{gb},{trap}} = {\frac{1}{e}\sqrt{8\varepsilon_{0}\varepsilon_{r}P_{net}{\Delta\varphi}_{gb}}}$

wherein ε₀ is the permittivity of free space, ε_(r) is a dielectricconstant of absorbing materials, Δφ_(gb) is the barrier height of bandbending, e is the elementary charge, and w is the grain boundary widthwhich was obtained from the surface potential line profile.

The maximum GB potential and defect density decreased from 35 meV and1.1342×10¹⁴ cm⁻³ for 200 mgml⁻¹ to 25 meV and 8.775×10¹³ cm⁻³,respectively, and reached the minimum at 300 mgml⁻¹. The grain boundarypotential and dopant density again increased to 41 meV and 6.5059×10¹³cm⁻³ while increasing the perovskite precursor concentration to 400mgml⁻¹. The perovskite film prepared with 100 mgml⁻¹ perovskiteprecursor showed comparable grain boundary potential with perovskitefilms prepared from 300 mgml⁻¹. However, the absorption of theperovskite film with 100 mgml⁻¹ was minimum which reduced thephotocurrent of the ML-perovskite sensor. Similarly, the density ofcharged trap states decreased from 1.5575×10¹¹ cm⁻² at 200 mgml⁻¹ to1.0863×10¹¹ cm⁻² and reached the minimum for 300 mgml⁻¹ and increasedagain to 1.2143×10¹¹ for 400 mgml⁻¹ perovskite film. This was consistentwith the highest device performance of the ML-perovskite sensor preparedfrom 300 mgml⁻¹ (see below).

Dynamic mechanical pressure sensing: FIG. 8A depicts the sensor responsedue to the applied pressure on the sensor. The current output increasedwith an increase of applied pressure. A regression model was applied tothe experimental data, and the fitted line exhibited linearity with aregression coefficient (R²) of 0.87. The sensor could sense a minimumpressure of 25 kPa with a current output of 1 μA. In order to preventsensor breakage, the applied pressure was limited to 500 kPa. Thepressure sensitivity S of the sensor was 0.0115 μA/kPa, which was theslope of the regression line. Mechanical energy stimulated the releaseof trapped electrons in ZnS:Cu, which eventually emitted as light with awavelength of 543 nm.

When higher mechanical energy was applied to ZnS:Cu crystals, the lightemitted increased, which results in a higher electrical current. FIG. 8Bdepicts a current vs. time plot of the sensor response with variousapplied pressures. The pressures were applied to the sensor three times.The curves at a particular pressure were almost identical. The sensordisplayed linearity, which may be an important characteristic of asensor.

Also plotted was a 40 cycle sensor response due to the cyclic bendingtest. The sensor was subjected to 1 mm vertical displacement andgenerated distinct electrical signals for each cycle. Such high currentoutput from the sensor due to small mechanical deformation demonstratedthat the sensor had high sensitivity, which may be a desirablecharacteristic of any pressure sensor. The sensor's output current wasalmost constant at a value of 7.35 μA at the peak of each cycle. Thesignal consistency demonstrated that the sensor had the potential forlong-term performance capability without degradation.

FIG. 8C depicts the sensor's response due to a sudden applied pressureon the sensor. The sensor was connected to a current-to-voltageamplifier to measure the response time of the sensor. The data from theamplifier were collected using a NI-6210 DAQ with a sampling rate of 200kHz. The sensor displayed a response time of 25 μs, which was limited bythe amplifier. The amplifier had a response time of 17.5 μs at frequencybandwidth of 20 kHz and a current-to-voltage conversion factor of 1V/μA. It has been demonstrated that perovskite-based photodetectors havean ultrafast response time in the nanosecond range with operation atzero bias. Apparently, the amplifier and the data acquisition systemlimited the response time measurement of the sensor. Therefore, theresponse time of the sensor was less than 25 μs.

FIG. 9 depicts the response of the sensor of this example to a gentlefinger touch. The sensor was subjected to touch at 26 times by indexfinger and the sensor generated distinct significant electrical signalscorresponding to each soft touch. As such, the sensor has the potentialto play a role in at least the following pressure sensitiveapplications: (i) touchpad, (ii) robotic arm grip, (iii) e-skin, and(iv) seat occupancy detection in trains or public transports.

For example, a sensitive pressure sensor could be used to detect thepatient positioning to prevent pressure ulcer formation on foot. TheML-perovskite sensor was attached to an insole of a shoe and appliedpressure on the sensor by foot. The sensor response to the pressureapplied by foot was plotted, and the results showed that the sensorsgenerated high signals at high pressure regions and low electricalsignals at low pressure regions.

As such, the sensor could be utilized to determine which areas of thepatient are being exposed to prolonged pressure by tracking patientposition over time. As the sensor had the ability to generate signalsdue to varying pressure, the sensor has the potential for ulcerdetection on a patient's feet by embedding or attaching a pressuresensor in socks or insoles.

The sensor was attached to a carbon fiber reinforced composite (CFRP)beam, and a cyclic 3-point bending test was performed. The sensorresponse to 1000 cycles was plotted, and the sensor displayed aconsistent current output of 0.45 nA at the peak over the cycles, whichdemonstrated its capability of long-term performance in structuralhealth monitoring.

Example 4—Flexible and Bendable Pressure

In this example, an efficient mechanoluminescent light harvesting methodwas designed by integrating a perovskite material into the ML device.This fully integrated mechanoluminescent-perovskite device utilized avery thin, flexible, and highly transparent substrate (transmittancearound 90%) to minimize the loss of light during transmission. Thepressure sensor of this example had a vertical type architecture similarto FIG. 1C with the following layers:

EuD₄TEA-PDMS/PET/ITO/SnO₂/perovskite/Au.

The use of EuD₄TEA and the thin, highly transparent PET substrate likelycontributed, at least in part, made the sensor sensitive and efficient.

Sample fabrication: EuD₄TEA crystals were thoroughly mixed with apolydimethylsiloxane (PDMS) elastomer with a mixer. The ratio of EuD₄TEAcrystals and PDMS was 1:2. Thin films were prepared on glass substrateswith drop casting and cured at 75° C. for 2 hours. Since EuD4TEAcrystals started to break down at 100° C., the curing temperature of 75°C. was selected. The PET substrates were treated (surface treatment)with an oxygen plasma etching machine to obtain a rough surface.EuD₄TEA/PDMS composite was drop casted on the PET substrate.

Sensor performance before, during, or after stretching and bending: Thethin films were tested by stretching them 5 times and a PMT was used tocollect light from the thin films. FIG. 10A shows the ML intensity ofEuD₄TEA/PDMS composite thin film when stretched 5 times and in thephotoluminescence of EuD₄TEA/PDMS composite thin film under 365 nm UVlight was observed.

The thin films on PET substrate were tested by elastically bending them3 times and a corresponding ML intensity from the film was plotted, asdepicted at FIG. 10B. Three distinct peaks were observed thatcorresponded to each bending. The intensity varied due to the unequalbending radius. As such, the ML intensities differed from each other.The photoluminescence of EuD₄TEA/PDMS composite thin film under 365 nmUV light also was observed. These preliminary tests demonstrated thatthe EuD₄TEA/PDMS was an excellent candidate for integratedmechanoluminescent-perovskite flexible, bendable pressure sensor.

Device architecture: Again, the device of this example had anarchitecture similar to the schematic of FIG. 1C. The devicearchitecture adopted a vertical type structure that included a thinflexible EuD₄TEA/PDMS film (140 of FIG. 1C) on a transparentpoly(ethylene terephthalate) (PET) substrate (150 of FIG. 1C) and 3DCH₃NH₃PbI₃ and mixed cations FA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃perovskite (120 of FIG. 1C).

The EuD₄TEA crystals emitted ML light of 626 nm, i.e., 1.98 eV. TheCH₃NH₃PbI₃ perovskite had a band gap energy of 1.5 eV, which was lowerthan 1.98 eV. Similarly, the FA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃perovskite had a band gap below 1.98 eV. As such, in this example, 3DCH₃NH₃PbI₃ was used as the baseline andFA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ was used as an absorbing layerfor the device. The use of FA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃perovskite likely made the sensor stable and highly sensitive.

The integrated structure was effective for sufficiently absorbing MLlight from EuD₄TEA with minimum transmission loss. ML-based sensors suchas ITOF and ITOFPress sensors adopted end coupling design, which havehigher transmission loss. A highly uniform perovskite active absorberlayer were deposited on an electron transporting layer tin (IV) oxide(SnO₂) layer (160 of FIG. 1C). Atop the perovskite layer, a 100 nm thickAu (110 of FIG. 1C) was deposited by thermal evaporation. The emittedlight from EuD₄TEA crystals passed through the transparent PET substrateto the perovskite layer.

Potential applications: This sensor of this example has potentialapplications in health monitoring, e-skin, touchpad, etc. In order touse as health monitoring, the sensor may be bendable to conform to thewrist or other body part.

FIG. 11 depicts of an embodiment of a bendable device architecture forwrist band, which has no rigid substrate. First, a thin layer ofPDMS-EuD₄TEA film (1105) was prepared and a thin Ag coating (1106) wasdeposited on its surface. The opposite side of the Ag coating (1106) wasfirst coated with ITO (1104) and followed by a SnO₂ layer (1103). A 3Dmixed perovskite (1102) was deposited on the SnO₂ (1103) surface andfinally, a 100 nm Au coating (1101) was deposited using thermalevaporation.

Another sensor 1200 was made, as depicted at FIG. 12 , which may havepotential applications in artificial electronic skins, structural healthmonitoring, energy harvesting, ulcer detection on foot, prosthetics,soft robotics, wearable technology, health monitoring, touchpad, etc.

In order to use a sensor for health monitoring, the sensor needs to bebendable to conform to the wrist or other body part. FIG. 12 depicts aschematic of the bendable device architecture of this example, which hadno rigid substrate, thereby making it a suitable sensor for conformingto the wrist or other body parts. First, a thin layer of PDMS-EuD₄TEAfilm (1206) was prepared and a thin Ag coating (1207) was deposited onthe surface. The opposite side of the Ag coating (1207) was first coatedwith ITO (1205) and followed by a SnO₂ layer (1204). A 3D mixedperovskite (1203) was deposited on the SnO₂ (1204) surface and finally,a 100 nm Au coating (1201) is deposited using thermal evaporation on alayer of Spiro-OMeTAD (1202).

Example 5—Integrated Mechanoluminescent-Perovskite Self-Powered FlexibleSensor for Dynamic Pressure Sensing

In this example, a sensitive pressure sensor was fabricated by fullyintegrating an ML device with a mixed halide perovskite. The devicearchitecture of this example had a vertical type structure, which likelycontributed, at least in part, to its comparatively easy fabrication andefficient ML harvesting.

FIG. 1 presents a schematic diagram of flexible pressure sensorarchitecture, band energy diagram, and fabrication process. The devicearchitecture, as depicted at the schematic of FIG. 13 , had a verticaltype structure of “ZnS:Cu-PDMS (1307)/PET (1306)/ITO (1305)/SnO₂(1304)/perovskite (1303)/Spiro-OMeTAD (1302)/Au (1301)”, which includedtwo primary functional layers (i) a thin flexible copper-doped zincsulfide (ZnS:Cu)/PDMS film as a light emitter and (ii) a mixed halideperovskite CH₃NH₃Pb(Br_(0.1)I_(0.9))₃ as a light absorber.

Tin (IV) oxide (SnO₂) and 2,2′,7,7′-Tetrakis [N, N-di(4-Methoxyphenyl)Amino]-9,9′-spirobifuorene (Spiro-OMeTAD) were used as the electrontransporting material and hole transporting material, respectively. A200 μm thick flexible, translucent ITO-coated poly(ethyleneterephthalate) (PET) was used as a substrate, and a 100 nm thick Au wasused as an active photocathode.

According to a band energy diagram of the device of this example, ZnS:Cuemitted light due to an applied pressure or strain, which passed throughthe translucent SnO₂-coated ITO/PET substrate to the perovskite absorberlayer, which created a free electron and hole in the material. Theelectron and hole then move towards the cathode (ITO) and anode (Au),respectively. An overlay of the UV-vis spectrum of thin filmMAPb(Br_(0.1)I_(0.9))₃ perovskite and ML emission of ZnS:Cu wascollected. MAPb(Br_(0.1)I_(0.9))₃ perovskite had a range of absorptionfrom about 400 nm to about 770 nm and the ZnS:Cu crystals emitted MLlight centered at about 543 nm (2.28 eV). Stable and bright greenluminescence is ascribed from the copper atom introduced energylevel.^([29]) ML emission of ZnS:Cu emerged from the recombinationbetween the impurity-induced shallow donor state and the t₂ state of Cu.

The band gap determination was made from Tauc plot. The calculatedoptical band gap (E_(g)) of MAPb(Br_(0.1)I_(0.9))₃ perovskite was 1.64eV, which was much lower than 2.28 eV. Therefore, the perovskitematerial could fully absorb ML emission from ZnS:Cu. The transmittanceof PET/ITO/SnO₂ is 79% at 543 nm was determined. Unlike other ML-basedsensors, the integrated structure was effective for sufficientlyabsorbing ML light from ZnS:Cu with minimum transmission loss. An atomicforce microscopy (AFM) image demonstrated a smooth, homogenous SnO₂layer on the ITO/PET substrate, which likely was beneficial to thecrystallization of the perovskite film.

High-quality perovskite films likely contributed significantly to theoptoelectronic device performance. The processing of the perovskite hada significant effect on film morphology, uniformity, and crystallinityof the perovskite films, thereby resulting in an impact on the deviceperformance. SEM images showed compact perovskite thin films completelycovering the surface of the PET/ITO/SnO₂ substrate. SEM image confirmedthe pinhole-free surface morphology and enhanced grain size ofMAPb(Br_(0.1)I_(0.9))₃ perovskite. The average grain size of theperovskite was 282 nm with a range of about 50 to about 700. Largergrain sizes in perovskite films possessed fewer grain boundaries,thereby likely resulting in reduced defect density. The charge carriersencountered less trapping in grain boundaries that promoted theefficiency of the device. A cross-sectional SEM image of the perovskitefilm was collected. The average thickness of the perovskite film wasabout 500 nm. X-ray diffraction (XRD) patterns of MAPb(Br_(0.1)I_(0.9))₃perovskite film and PET substrate were collected. The breakregion)(16-28.4° in the XRD pattern sought to remove the strongdiffraction from the PET substrate. Three dominant crystallographicplanes (100), (200), and (210) were observed at the peak of diffractionangles of 14.2°, 28.6°, and 32°, respectively, which confirmed the cubichalide perovskite crystalline structure.

The time-resolved photoluminescence of perovskite film was analyzed, andthe values of fast decay, τ₁ and slow radiative decay, τ₂ were 1.07 nsand 14.9 ns, respectively, for MAPb(Br_(0.1)I_(0.9))₃ perovskite film ontop of SnO₂.

An SEM image of the as-received ZnS:Cu crystals revealed an averageparticle size of about 8.91 and powder XRD performed on the as-receivedZnS:Cu crystals revealed that the cubic phase was depicted with peaks at28.57° (111), 47.49° (220), and 56.37° (311). No other diffraction peakof impurity was found in the XRD patterns.

A high-resolution transmission electron microscopy (HRTEM) image ofZnS:Cu was collected, and a selected area electron diffraction (SAED)pattern quantitatively confirmed the crystalline nature of the crystal.Higher crystallinity of crystals were believed to result in enhanced MLintensity. A cross-sectional SEM image of a ZnS:Cu/PDMS film showedZnS:Cu particles surrounded by the PDMS matrix, and no large particleclusters. A photograph of ZnS:Cu/PDMS thin film was taken under a UV 365nm light, and the ZnS:Cu emitted light of a bright green color.

In order to investigate the sensor response to applied pressure, sensorswere tested by applying normal pressure. FIG. 14A displays the responseof the sensor to continuous linearly applied pressure. The pressing ratewas 250 N min′. The pressing rate was defined as how fast the pressurewas applied to the sensor. The current output increased with an increasein applied pressure. A regression model was applied to the experimentaldata, and the fitted line exhibited linearity with a regressioncoefficient (R²) of 0.9907. The regression model for the data is shownin the following equation:

$\frac{\Delta I}{I_{0}} = {{{0.0}95P} + {{1.1}7}}$

wherein ΔI is the relative change of current, and h is the currentduring no pressure.

The sensor showed a linear relationship to the applied pressure. Ingeneral, the ML materials showed a linear relationship due to theapplied pressure on the ML crystals/composites.

The ML relationship was expressed as follows for a particular pressingrate: I_(ML)∝P; wherein I_(ML) is the ML intensity from the crystals andP is the applied pressure.

The photocurrent of a perovskite photodetector increased linearly withthe increase of light intensity. Again, the ML intensity increasedlinearly with the increase of applied pressure. Therefore, it waspostulated that the output signal of the system should increase linearlywith the increase of applied pressure. The mechanical energy (i.e.,applied pressure) stimulated the release of trapped electrons in ZnS:Cu,which eventually emitted as light with a wavelength of 543 nm. Thegreater the mechanical energy applied to ZnS:Cu crystals, the higher theML emission, which resulted in a higher electrical current (sensoroutput). The sensitivity of a pressure sensor was defined as the changeof sensor output, in this case, electrical current, due to the inputparameter change (pressure). In other words, sensitivity was the slopeof the output characteristics curve. The sensitivity S of the sensor isdefined as:

$S = \frac{\frac{\Delta I}{I_{0}}}{\Delta P}$

wherein ΔP is the change of applied pressure. The sensitivity of thesensor was 0.095 kPa⁻¹, which is the slope of the regression line ascalculated from FIG. 14A.

FIG. 14B depicts a current response versus time plot of the response ofthe response to various applied pressures. Again, the sensordemonstrated a linear relationship, which was an importantcharacteristic of a sensor. The linear increase of sensor's electricalcurrent output with an applied pressure without requiring anyirradiation for ML recovery indicated that the self-recovery of the MLin crystals likely took place by trapping of drifting charge carriers inthe presence of a piezoelectric field. As such, this crystal can besuitable for use as, or as part of, a durable sensor.

In order to examine the response of the sensor to mechanical bending,the sensor was loaded vertically as shown at FIG. 5 . Under a verticaldisplacement of 1 mm, a repeated bending test of 10 cycles was performedand plotted. The sensor generated distinct signals for each cycle. Thesensor response showed more or less peak current consistency over allcycles. In addition, the sensor output was consistently followed bymechanical input.

The response time was an important figure of merit for a pressuresensor. A faster response may be needed for accurate and precisemeasurement in certain applications. The response time of the sensor toapplied quick pressures were plotted, along with the correspondingimpact-induced sensor response. The response time of the sensor wasabout 25 μs.

The response of the sensor to 1000 cycles of a repeated 3-point bendingtest was determined. The sensor generated distinct significantelectrical signals corresponding to each bending cycle. During themechanical bending, the ZnS:Cu emitted light due to the strain, whichwas eventually converted to electrical current by integrated perovskite.The sensor showed a steady response over the cycles, which demonstratedits capability of long-term performance in structural health monitoring.

In this example, ITO-coated PET substrates were patterned by etchingwith hydrochloric acid and zinc powder. PET substrates were successivelycleaned with nano pure water, acetone, and isopropanol. The ITO-coatedconductive sides of the flexible PET substrates were treated (surfacetreatment) with an oxygen plasma etching machine to obtain enhancedwettability. An SnO₂ colloid precursor was diluted by deionized H₂O (1:6volume ratio) and stirred overnight. SnO₂ was spin-coated onto theconductive side (ITO-coated side) of the PET substrates at 3000 rpm for30 seconds and annealed. On top of SnO₂, the perovskite thin film wasdeposited with a one-step deposition method referred to as asolvent-solvent extraction method.

The precursor solution was prepared by mixing CH₃NH₃I into 0.5 mL ofN-Methyl-2-pyrrolidone (NMP) and 0.1 mL γ-butyrolactone (GBL). A mixtureof PbBr₂ and PbI₂ was poured into the previously prepared solution andmixed it on hot plate at 65° C. for 2 hours. The solution was heated onhot plate at 70° C. for 30 mins. The perovskite precursor solution wasspin-coated.

The perovskite-coated substrates were quickly immersed into diethylether (C₂H₅OC₂H₅, DEE) bath for 2 mins. Within this time frame, NMPsolvent extraction and a complete crystallization occurred and formed auniform, ultra-smooth perovskite film. The NMP was highly miscible indiethyl ether, as such DEE selectively extracts NMP solvent fromdeposited perovskite thin films and leaves devoid areas where perovskitecrystallization rapidly occurred. The perovskite was insoluble indiethyl ether, therefore the DEE did not dissolve crystallized films.

The perovskite films were thermally annealed at 70° C. for 5 minutesfollowed by 100° C. for 5 minutes in a nitrogen-filled glove box.Spiro-OMeTAD was spin-coated on the perovskite layer at a spin rate of2000 rpm for 60 s. The Spiro-OMeTAD solution was prepared by mixing 17.5μL of Li-TFSI solution (520 mg of Li-TSFI in 1 mL of acetonitrile), 28.8μL of 4-tert-butylpyridine, and 29 μL of Co (III) TFSI solution (300 mgof Co (III) TFSI in acetonitrile) with 90 mg of Spiro-OMeTAD in 1 mL ofchlorobenzene solution. A 100 nm of Au was deposited by thermalevaporation using an in-house-made shadow mask to pattern the electrode.ZnS:Cu was thoroughly mixed with a polydimethylsiloxane (PDMS) elastomerwith a mixer. The ZnS:Cu/PDMS composite was spin-coated on thenon-conductive side of a flexible PET substrate to obtain a uniform thinfilm. The casted thin film was heated to complete cure. Perovskite andZnS:Cu Characterizations: UV-vis absorption spectra were recorded on aCary 5000 UV-Vis FM-NIR spectrophotometer (Agilent, USA) in the 400-850nm wavelength range at room temperature. The X-ray powder diffractiondata on ZnS:Cu powder was collected using a PANalytical X-pert ProPowder XRD machine. Transmission electron microscopy (TEM) was used toanalyze the structure of ZnS:Cu crystal. Scanning electron microscopy(Zeiss 1540EsB field emission SEM) was used to characterize the surfacemorphology of perovskite films and ZnS:Cu particles. XRD patterns ofthin perovskite films were collected using a Scintag XRD PowderDiffractometer with Cu Kα radiation (λ=1.5406 Å). The time-resolvedphotoluminescence was performed at room temperature using an Edinburghinstruments FS5 spectrometer. Samples were excited by picosecond pulsedlight emitting diode (EPLED-365) with an excitation wavelength of 470nm. The time-resolved photoluminescence was measured using timecorrelated single-photon counter, excited by a picosecond pulsed diodelaser (EPL-470 nm). ML emission from ZnS:Cu crystals was recorded with aH-micro series spectrometer (C12880MA). AFM from Veeco Instruments Inc.,USA was used to characterize the surface morphologies of SnO₂.

Device Characterization and Testing: The current-voltage (I-V)characteristics of the photodetectors were recorded by using aKEITHLEY™-2410 source-measure unit (SMU). Mechanical testing ofML-perovskite sensor was performed by using a DMA Q800 from TAInstruments. An electrical insulating tape was attached to both top andbottom DMA fixtures to prevent from the short circuit as the DMAfixtures are conductive (made of metal). The cyclic 3-point bending testwas performed by utilizing a Shimadzu mechanical testing system. Theoutput current from the sensor was collected with a KEITHLEY™-2410 and acustom-built LAB VIEW™ program. To measure the response time of thesensor, an amplifier (C7319) from Hamamatsu was used to convertcurrent-to-voltage, and a NI-6210 DAQ was connected to the amplifier tocollect data. The response time of the flexible pressure sensor wasmeasured using a drop tower developed in-house for impact loading andthe data was recorded using a MATLAB program.

We claim:
 1. A mechanoluminescent device having a vertical devicearchitecture, the device comprising: an electrode; a first layercomprising a perovskite; a counterelectrode; a second layer comprising amechanoluminescent material and a matrix material in which themechanoluminescent material is dispersed; (i) a first chargetransporting layer, (ii) a first charge blocking layer, or (iii) a firstcharge transporting layer and a first charge blocking layer; (a) asecond charge transporting layer, (b) a second charge blocking layer, or(c) both a second charge transporting layer and a second charge blockinglayer; wherein the first charge transporting layer, the first chargeblocking layer, or both the first charge transporting layer and thefirst charge blocking layer is arranged between the first layer and thecounterelectrode, wherein the second charge transporting layer, thesecond charge blocking layer, or both the second charge transportinglayer and the second charge blocking layer is arranged between the firstlayer and the electrode, wherein the second charge transporting layer isin contact with the electrode, and wherein the first layer is arrangedbetween the electrode and the counterelectrode, and the counterelectrodeis arranged between the first layer and the second layer.
 2. Themechanoluminescent device of claim 1, further comprising a substratearranged between the counterelectrode and the second layer, wherein thesubstrate is transparent, flexible, or a combination thereof.
 3. Themechanoluminescent device of claim 1, wherein the first chargetransporting layer is in contact with the counterelectrode.
 4. Themechanoluminescent device of claim 1, further comprising a third layercomprising one or more reflector materials, wherein the second layer isarranged between the counterelectrode and the third layer.
 5. Themechanoluminescent device of claim 1, wherein the perovskite comprises aRuddlesden-Popper layered perovskite, an organo-metal halide perovskite,a mixed cation perovskite, or a combination thereof.
 6. Themechanoluminescent device of claim 1, wherein the mechanoluminescentmaterial comprises (i) zinc sulfide doped with copper, manganese, or acombination thereof, (ii) europium tetrakis dibenzoylmethidetriethylammonium, or (iii) a combination thereof.
 7. Themechanoluminescent device of claim 6, wherein the matrix materialcomprises polydimethylsiloxane (PDMS), poly(methyl methacrylate) (PMMA),polystyrene, polycarbonate, polyurethane (PU), polyvinylidene fluoride(PVDF), or a combination thereof.
 8. An article comprising themechanoluminescent device of claim
 1. 9. The article of claim 8, whereinthe article is a wearable article or a prosthetic.